1. Field of the Invention
This present invention is directed to an apparatus and system for deriving a desired biologic constituent concentration value present in a pulsatile flowing fluid, and is more particularly directly to such derivations in pulsatile flowing blood in a hemodialysis environment.
2. Background Art
Kidneys are located on either side of the spine. In a healthy patient, kidneys function to stimulate red blood cell production and regulate the content of the blood. Kidneys also produce hormones that affect other organs and control growth. When functioning properly, kidneys serve as a means for cleaning the blood by removing excess fluids and toxins. The filtering task in each kidney is performed in part by the some one million nephrons in the kidney. The nephrons are filtering units made up of tiny blood vessels. Each such blood vessel is called a glomerulus. Every day, roughly 200 quarts of blood and fluids will be processed by the kidney. The kidney removes about two quarts of water and toxic chemicals which are sent to the bladder as urine for subsequent voiding thereof by urination.
A patient whose kidneys are performing substandardly may be dialyzed as a substitute for the blood cleansing function normally performed by properly functioning kidneys. Dialysis is a process by which the function of the kidney of cleaning blood is substitutionarily performed. The process of dialysis was perfected for routine use in the 1960's, having been invented some 50 years ago. For the purposes of discussion and illustration of hemodialysis, FIG. 1 is now referred to. While FIG. 1 incorporates a view of a presently preferred embodiment of the present invention, it also incorporates a view of some common components which are typical in a general hemodialysis environment. The general environment of hemodialysis and typical components therein will now be discussed.
In hemodialysis, blood is taken out of a patient 200 by an intake catheter means, one example of which is shown in FIG. 1 as an input catheter 122. Input catheter 122 is intravenously inserted into patient 200 at a site 180 and is used for defining a blood passageway upstream of a blood filter used to filter the impurities out of the blood. The blood filter is also called a dialyzer 130. The unclean blood flows from an artery in patient 200 to a pump means, an example of which is pump 140. From pump 140, the blood flows to dialyzer 130. Dialyzer 130 has an input port 230 and an output port 240. The pump 140 performs the function of moving the unclean blood from patient 200 into input port 230, through dialyzer 130, and out of dialyzer 130 at output port 240.
Specifically, unclean blood in input catheter 122 is transported to input port 230 of dialyzer 130. After passing through and being cleansed by dialyzer 130, the blood may receive further processing, such a heparin drip, in hemodialysis related component 300. The now clean blood is returned to patient 200 after the dialyzing process by means of an output catheter means, an example of which is output catheter 124. Output catheter 124, which is also intravenously inserted into patient 200 at site 180, defines a blood passageway which is downstream from dialyzer 130, taking the blood output by dialyzer 130 back to patient 200.
As mentioned, the hemodialysis process uses a blood filter or dialyzer 130 to clean the blood of patient 200. As blood passes through dialyzer 130, it travels in straw-like tubes (not shown) within dialyzer 130 which serve as membrane passageways for the unclean blood. The straw-like tubes remove poisons and excess fluids through a process of diffusion. An example of excess fluid in unclean blood is water and an example of poisons in unclean blood are blood urea nitrogen (BUN) and potassium.
The excess fluids and poisons are removed by a clean dialysate liquid fluid, which is a solution of chemicals and water. Clean dialysate enters dialyzer 130 at an input tube 210 from a combined controller and tank 170. The dialysate surrounds the straw-like tubes in dialyzer 130 as the dialysate flows down through dialyzer 130. The clean dialysate picks up the excess fluids and poisons passing through the straw-like tubes, by diffusion, and then returns the excess fluids and poisons with the dialysate out of dialyzer 130 via an output tube 220, thus cleansing the blood. Dialysate exiting at output tube 220 after cleansing the blood may be discarded.
In sum, unclean blood flows from an artery in patient 200 to pump 140 and then to dialyzer 130. Unclean blood flows into dialyzer 130 from input catheter 122 and clean blood flows out of dialyzer 130 via output catheter 124 back to patient 200.
Hemodialysis, which removes excess fluids from the blood of a patient, has an acute impact on the fluid balance of the body due in part to the rapid change in circulating blood volume. When the fluid removal rate is more rapid than the plasma refilling rate of the body, the intravascular blood volume decreases. This resulting fluid imbalance has been linked to complications such as hypotension, loss of consciousness, headaches, vomiting, dizziness and cramps experienced by the patient, both during and after dialysis treatments. With hypotension and frank shock occurring in as many as 25% of hemodialysis treatments, dialysis induced hypovolemia remains a major complication of hemodialysis.
Many dialysis patients already have compromised circulatory responses due to the secondary effects of the end stage renal disease. A malfunctioning of the blood pressure compensatory mechanisms due to intravascular volume depletion has been considered to be one of the major factors causing dialysis induced hypotension.
In order to reduce the chance of dialysis induced hypotension, continuous measurement of the circulating blood volume can optimize dialysis therapy regimes, control the fluid balance, and aid in achieving the dry weight goal of the patient on a quantitative basis. Volumetric controllers, while giving a precise measurement of the amount of fluid removed through ultrafiltration, do not give any indication of how the plasma refilling mechanisms of the body are responding to the actual fluid removal. Factors such as food and water intake and postural changes also significantly effect the circulating blood volume during dialysis. Maneuvers such as eating, drinking and posture, illustrate how sensitive the plasma refilling mechanisms are.
The hematocrit value gives an indication of blood volume change. Since the number of red blood cells in whole blood is not significantly altered by dialysis, and the mean corpuscular volume of the red cells remains essentially constant, it follows that the changes in blood volume will be inversely proportional to the changes in hematocrit. Therefore, blood volume change of the patient may be defined at any time during the course of dialysis treatment as in EQUATION 1. ##EQU1## Where: BV.sub.final =Final Blood Volume
BV.sub.initial =Initial Blood Volume PA1 HCT.sub.final =Final Hematocrit Value PA1 BV.sub.i =Initial Blood Volume PA1 HCT.sub.i =Initial Hematocrit Value PA1 HCT.sub.f =Final Hematocrit Value
In the clinical setting, however, it may be more useful to determine the percentage of blood volume change as represented by EQUATION 2. ##EQU2## Where: BV.sub.f =Final Blood Volume
It is known to use hematocrit change as a measure of the actual blood volume change occurring during dialysis. However, in order that the relationship between hematocrit change and blood volume change to be useful, the hematocrit must be monitored accurately, continuously and in real time during the entire hemodialysis treatment session. While accuracy may be achieved through elaborate technical means, to be clinically practical, a real time hematocrit and blood volume monitor should be easy to use, save nursing staff time, operate noninvasively and be justifiable on a cost basis.
Various techniques employed to monitor intravascular blood volume change, due to ultra filtration, as a function of hematocrit value include microcentrifugation, electrical conductivity, and photometry.
In microcentrifugation, a microcentrifuge is used to measure hematocrit. This process is inadequate for monitoring real time changes in blood volume, due to the amount of time that elapses between measurements, the large potential for reader and sampling error, and the need to compensate appropriately for trapped plasma in the red cells columns. Hence, because of the labor intense nature of centrifuging the blood samples of the patient on a timely basis, this technique is wholly inadequate, impractical, and far too costly for wide scale clinical application.
In an attempt to achieve real time hematocrit information, electrical conductivity measurements have been used. Conductimetric measurements, however, are adversely affected by abnormal electrolyte, anticoagulant, and protein concentrations, all of which are prevalent among dialysis patients. Hence, this particular technique is fraught with significant technical errors as well.
Optical techniques, while generally unaffected by the above problems, have been susceptible to other instabilities. These include ambient light variations, tubing artifact, changes in blood flow rate, in-line pressures, and oxygen saturation. Additionally, the light sources used in optical techniques require frequent calibration.